1. Field of the Invention
Embodiments of the invention relates to coating, and more particularly, to an apparatus and method for coating a polyimide layer.
2. Background of the Related Art
A liquid crystal display (LCD) device is an apparatus for displaying a desired image by adjusting quantity of light reaching a color filter substrate. The adjustment of the quantity of light is accomplished by changing intermolecular orientation of liquid crystal molecules interposed between a transparent insulating substrate serving as the color filter substrate and an array substrate. One type of LCD device is a thin film transistor liquid crystal display (TFT LCD) device, which uses thin film transistors (TFTs) as switching elements.
In general, an LCD device includes an LCD panel for displaying an image and a driver for driving the LCD panel by applying driving signals to the LCD panel. The LCD panel includes a color filter substrate and an array substrate bonded to each other with a predetermined gap therebetween. A layer of liquid crystal molecules is in the gap between the color filter substrate and the array substrate. The color filter substrate and the array substrate of the LCD panel are manufactured through a plurality of masking processes. Polyimide layers are formed on respective substrates after finishing the masking processes and before the substrates are bonded to each other. The polyimide layers are used as alignment films to arrange the liquid crystal molecules in a predetermined direction.
The polyimide layers can be coated on the substrates through a variety of methods, such as a spin-coating method, a spray-coating method, and an inkjet-coating method. Of the coating methods, the inkjet-coating method is the quickest and easiest to apply because of the use of an inkjet coating apparatus. A plurality of inkjet heads are used in an inkjet coating apparatus to jet polyimide liquid onto the substrates.
FIG. 1A and FIG. 1B are schematic views illustrating an accumulation problem in an apparatus for coating a polyimide layer according to the related art. More specifically, FIG. 1A and FIG. 1B are schematic views illustrating the surface flatness of the inkjet head 10 before and after jetting polyimide liquid onto the substrates in the inkjet-coating method according to the related art. The inkjet head 10 moves in a predetermined direction above a print table on which a target substrate is placed while jetting polyimide liquid onto the target substrate, thereby coating a polyimide layer onto the substrate. As shown in FIG. 1A, when the jetting surface of the inkjet head 10 is clean and does not have any residue thereon, polyimide liquid can be jetted uniformly so that a polyimide layer having uniform thickness can be formed on the substrate. However, as shown in FIG. 1B, polyimide liquid residue can accumulate on the jetting surface A1 of the inkjet head 10 even after only one inkjet coating process.
The accumulated polyimide liquid residue on the jetting surface of the inkjet head 10 interferes with jetting of the polyimide liquid during a subsequent inkjet coating process. Thus, a non-uniform polyimide layer may be coated on a substrate or, in extreme cases, no polyimide layer is coated on a substrate. The non-uniform polyimide layer can include pinhole faults and/or line blemishes. Even if the jetting surface of the inkjet head is cleaned using a cleaning bar, polyimide residue may still remain on the inkjet head.